Solution
= Solution
For the <normal linear model>, the <maximum-likelihood estimators> are
$$
\boxed{\widehat\beta=(X^TX)^{-1}X^TY}
$$
and
$$
\boxed{
\widehat\sigma^2
=\frac1n\|Y-X\widehat\beta\|^2
}.
$$
The divisor is $n$ for maximum likelihood, rather than the degrees-of-freedom divisor used by the unbiased residual-variance estimator.